I needed to insert new line after existing one in multiple similar xml files.
In Visual Studio open "replace in files" dialog and tick "use regular expression"
In "find" escape "/" as "\/"
<param name="ExistingKey" value="False" \/>
In "Replace" repeat original string, then add \n and add a new string
<param name="ExistingKey" value="False" />\n <param name="NewKey" value="new value" />
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